Modeling of Electrochemical Remediation

Mathematical models are useful to better understand the processes that occur under electric field and predict remedial performance in field application. Compared with laboratory studies, only few studies have been reported on the mathematical modeling of electrochemical processes and remediation. Generally, electrochemical remediation models should incorporate the contaminant transport, transfer, and transformation processes and dynamic changes in electrical conductivity, pH, and geochemical reactions. Recognizing this as a complex task, researchers have developed some simple models based on a set of simplified assumptions (Chapters 25 and 26). 

The classical H-S equation is used to predict the electro-osmotic velocity of the fluid as a function of the electric field and the electrokinetic potential of the clay. Both of these parameters vary during electrokinetic transport, and result in a nonlinear process. New models have been developed that uncouple the electro-osmotic velocity from the applied field taking that surface conductivity and the resulting proportion of the current transferred over the solid-liquid interface are used as intrinsic properties of the clay to describe the velocity (Chapter 2). The pH changes affect the zeta potential, and thereby electro-osmotic conductivity. Thus, electro-osmotic conductivity changes as the dynamic changes in soil pH occur. 

Modeling of electrolysis reactions and the corresponding changes in soil pH and geochemical reactions require knowledge of electrochemistry and geochemistry. The 

Examples of one-dimensional and two-dimensional models to predict the transport of heavy metals under constant DC current are explained in Chapter 25. This model ignores hydraulic advection, electrophoresis, diffusion, and electroosmosis processes and considers only electromigration. Electrical potential distribution is assumed to be a function of the electrical resistance of the soil and depends on the instantaneous local concentration and mobility of all the ions existing in the pore water of the soil. Local chemical equilibrium is assumed to calculate the concentration of chemical species. Validation of these and other developed models based on laboratory and field test results is critical to gain confidence in the accuracy of the model predictions. 

Mathematical models are also developed to assess the performance of electroki-netic barriers (Chapter 26). In general, hydraulic advection, electro-osmotic advection, and electromigration processes are incorporated in these models. Future work needs to incorporate biochemical reactions into the modeling of the behavior of electrokinetic reactive barriers. Similarly, a suite of mathematical models is needed for predicting the performance of other integrated electrochemical remediation systems. 

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